Gravity current

In fluid dynamics, a gravity current or density current is a primarily horizontal flow in a gravitational field that is driven by a density difference in a fluid or fluids and is constrained to flow horizontally by, for instance, a ceiling. Typically, the density difference is small enough for the Boussinesq approximation to be valid. Gravity currents can be thought of as either finite in volume, such as the pyroclastic flow from a volcano eruption, or continuously supplied from a source, such as warm air leaving the open doorway of a house in winter. Other examples include dust storms, turbidity currents, avalanches, discharge from wastewater or industrial processes into rivers, or river discharge into the ocean. Gravity currents are typically much longer than they are tall. Flows that are primarily vertical are known as plumes. As a result, it can be shown (using dimensional analysis) that vertical velocities are generally much smaller than horizontal velocities in the current; the pressure distribution is thus approximately hydrostatic, apart from near the leading edge. Gravity currents may be simulated by the shallow water equations, with special dispensation for the leading edge which behaves as a discontinuity. When a gravity current propagates along a plane of neutral buoyancy within a stratified ambient fluid, it is known as a gravity current intrusion. Although gravity currents represent the flow of fluid of one density over/under another, discussion is usually focused on the fluid that is propagating. Gravity currents can originate either from finite volume flows or from continuous flows. In the latter case, the fluid in the head is constantly replaced and the gravity current can therefore propagate, in theory, forever. Propagation of a continuous flow can be thought of as the same as that of the tail (or body) of a very long finite volume. Gravity flows are described as consisting of two parts, a head and a tail. The head, which is the leading edge of the gravity current, is a region in which relatively large volumes of ambient fluid are displaced.